The present invention relates to an anti-lock hydraulic brake system for automotive vehicles equipped with a pedal-actuated braking pressure generator. The system includes a master cylinder with a brake power booster inserted upstream thereof and a hydraulically controlled pressure modulator interconnected between the master cylinder and the brake power booster. An auxiliary-pressure source and a pressure-compensating reservoir are connected to said pressure modulator via electrically actuatable valves of a valve assembly. The pressure modulator serves to generate a resetting force opposing the pedal force, the resetting force reducing or compensating for the force which is transmitted to the master cylinder pistons on brake application. The system is further equipped with wheel valves which are inserted into the pressure-fluid conduits leading from the braking pressure generator to the wheel brakes individually or to the wheel brakes jointly. These wheel valves are controllable by an anti-lock controller and serve to shut off the pressure-field conduits.
A brake system of this general type is disclosed in German published patent application No. 33 17 629 wherein the braking pressure is controlled by a time-multiplex method for slip control. For this purpose, electromagnetically actuatable multidirectional control valves are inserted into the pressure-fluid conduits by which the wheel brakes are connected individually and/or in pairs to the braking pressure generator. These valves allow opening or closing of the pressure-fluid passage. The control valves also enable production of an auxiliary force which counteracts the pedal force and enables reduction of the force applied on the pistons in the master cylinder for slip control. Thus the braking pressure generated in the master cylinder is also reduced. When all the wheels run without locking, all the wheel valves (i.e., those valves inserted between the braking presure generator and the wheel brakes) are switched to open passage and thus communicate with the master cylinder. In the event of an imminent locked condition at any one wheel, the wheel valves leading to all of the other wheels are switched to closed positions for a short time so that only the pressure-fluid circuit of the imminently locking wheel is still connected to the master cylinder. Then, an auxiliary force is built up which acts, either partially or completed, against the (auxiliary-force-assisted) brake-pedal pressure whereby the braking pressure is reduced at the imminently locking wheel. The pressure remains constant in this phase in all of the other wheels. After the pressure has been reduced until attainment of the pressure level desired, the reduced pressure at the wheel which initially became unstable will be maintained constant by valve change-over. After the decrease of the counter force and the renewed pressure development in the master brake cylinder, the braking pressure build-up can be continued in the other wheel brakes. In this manner, the braking pressure at each wheel can be successively set to the desired value calculated by the associated electronics.
If, in the capacity of a pressure modulator, a hydraulically displaceable piston interposed between the master cylinder and the brake pedal or the brake power booster inserted upstream thereof is used in an anti-lock brake system for generating the brake force opposed to the pedal force, it must be assured that this piston will not prevent the transmission of the pedal force to the master cylinder when normal braking is performed (i.e., prior to commencement of slip control). This is because any inadvertent locking of the piston would render the brake out of action. Therefore, the working chamber into which auxiliary pressure is introduced for generating the force opposed to the pedal (i.e., the resetting force) must be open to a pressure-compensating reservoir in the normal case prior to commencement of anti-lock control.